Here are the corrected sentences with the appropriate forms of the given words:

1. Last year, many visitors **decided** to stay in homestays and learn the lifestyle of the Villa Gers.
2. AI like ChatGPT is believed to be **beneficial** in the search for information.
3. Minh Thu, the youngest **contestant**, won the special prize for "Promising Voice."

Here are the completed sentences using the provided root words:

1. Last year, many visitors **decided** to stay in homestays and learn the lifestyle of the Villa Gers. (DECISION)

2. AI like ChatGPT is believed to be **beneficial** in the search for information. (BENEFIT)

3. Minh Thu, the youngest **contestant**, won the special prize for "Promising Voice." (CONTEST)